Wnt proteins are one of several families of secreted growth factors that are specify cell fate during embryogenesis and renewal of tissues in the adult. Inappropriate activation of the pathway is associated with a number of cancers. Wnts bind to two co-receptors: the Frizzled proteins (Frz), and LDL-receptor related proteins 5 and 6 (Lrp5/6). Wnt-Frz-Lrp5/6 interactions are modulated by various activators and inhibitors, including the vertebrate Dickkopf (Dkk) proteins. In this proposal, we will use a combination of structural and biochemical approaches to define the molecular basis of Lrp5/6 and Frz interactions with Wnts and Dkks. 1. Crystal structures of human Dkk1 bound to fragments of Lrp6 will be determined. The contributions of different regions of these molecules to the interaction will be determined by calorimetry. These data will be used to assess whether the interaction with Dkk1 modulates the conformation of Lrp6 to render it inactive in Wnt signaling. 2. Structural and binding studies will be performed on Drosophila WntD and its receptors. PUBLIC HEALTH RELEVANCE: t proteins are secreted growth factors that are essential for the differentiation of stem cells. This proposal aims to understand the interactions of Wnts and their receptors. The molecular information from these studies could provide a foundation for rational, efficient development of agents that act on cancer stem cells.